64 ANNALS NEW YORE ACADEMY OF SCIENCES 



Eemarks : The shells included under this name form a perfect grada- 

 tional series from forms in which the loss of nodes appears only on the 

 later part of the body volution to those in which both nodes and spirals 

 are absent, if we include the forms figured by Deshayes, from nearly the 

 whole surface. So perfect is the gradation in the collection studied that 

 hardly two individuals are alike, and each may be considered a mutation, 

 though for convenience they are described under one name. 



The loss of ornamentation, the flattening of the whorls and the over- 

 lapping of the later whorls upon the earlier are all old-age features which 

 indicate a progressive gerontism and approaching extinction in the 

 branch of evolution which they represent. 



The life history of this species is closely similar to that of P. cordieri 

 mut. typicum, down to the stage when gerontic features begin to appear. 

 It differs from that species in having a wider apical angle, but the twa 

 species are doubtless closely related. 



Potamides lapidum Lamarck 



1804. Cerifhiinii lapidum Lamarck, Ann. du Mus. Nat. d'hist. naturelle de 



Paris, III, 350. 

 1824. Cerithwm lapidum Deshayes, Desc. des coquilles foss. des environs de 



Paris, II, 421. pi. 60. figs. 21-22. 

 1906. Potamides lapidum Cossmann. Essais de Pal^oconch. Comp., VII, 104, 



pi. 10, figs. 6-7. 



Measurements : Length, 31 mm. ; greatest diameter, 9.5 mm. ; apical angle, 

 18.5° ; sutural angle, 88°. 



The youngest volution studied is .8 mm. in diameter, and there are 

 evidently several volutions missing above this. Its ornamentation con- 

 sists of two very fine, equal, continuous spirals. On the next volution 

 these spirals are crossed by very faintly developed, oblique ribs which are 

 present with varying strength and frequency on the succeeding ten volu- 

 tions. These ribs are never well developed, and on the adolescent and 

 adult whorls they give place altogether to crowded lines of growth. On 

 the later volutions the spirals also become indistinct, and the whorls have 

 a rounded outline, with a surface roughened by the crowded lines of 

 growth and faint traces of one or two spirals. On the young of some 

 specimens the lower of the two spirals is more prominent than the upper, 

 and the ribs in crossing give it a nodose appearance, but this ornamenta- 

 tion also disappears from the later whorls. 



The aperture is circular, with a thickened callus on the inner lip, and 

 the margin of the outer lip is sinuous. The siphonal canal is shallow 

 and broad, with a strongly reflexed margin. 



